The control devices or actuators in a rolling mill influence the flatness of the strip in different ways by affecting the roll gap profile of the work rolls.
A condition for high performance flatness control is to have continuous access to the actual flatness across the strip, that is, a flatness profile. With a known flatness profile, the rolling mill can be provided with a flatness control system that based on the measured flatness profile and a given target or reference flatness profile computes set points to the available control devices, achieving closed-loop flatness control, see FIG. 1. The flatness control comprises several executing devices which means that a relatively complex evaluation process have to be done in order to decide on the magnitude of the various actions by the control devices, which provide the best result.
A measurement device could be designed as a measuring roll of metal, with something like 16-64 measuring points located across the strip, which in most cases can be placed between the mill stand and the wind-up reel without the use of deflector rolls. Such a measuring roll is the “Stressometer” produced by ABB. The measurement takes place with the aid of force transducers, based on e.g. the magnetoelastic principle, and primarily provides the stress distribution of the strip along the measuring roll. If the stress is greater than the buckling stress for the material, the sheet buckles when the strip is left free with no influence by any tensile force. The stress distribution is a flatness profile for the strip across the rolling direction. Depending on the technology of the flatness measuring device and the current rolling speed, a new complete flatness profile measurement across the strip may be obtained as often as every 4:th ms (millisecond).
When rolling a strip, it is important to maintain the desired flatness profile at all times. Deviation from the desired flatness may result in costly strip breaks. The task of the flatness control system is thus to drive the actual flatness profile as close possible to the desired flatness profile, which put high requirements on such a system, in terms of calculation speed and accuracy.